You might want to see if it's delimited, top speed is limited to about 115mph iirc on JDM's. Tax might be higher too, depends on how it was tested when it was registered in the UK. You'd want to check the mileage i'd think, mine is in kilometers up untill it was converted, then in miles after that so it's not a simple matter of reading the numbers off the dials. Also, nearly all imports suffer a busted splitter when they are unloaded from the ship, if it was imported in a containter you won't have that problem, but worth paying close attension to the front end bodywork and shut lines etc.
All i can think of right now, jdm's are very similar to UK's tbh, not 'huge' differences at all on these cars
ed: Just thought, the ecu's run a slightly leaner map due to better fuel in Japan, you might want to check that it's been run on v-power or similar.